1. Field of the Invention
The present invention relates to a power transmission device such as a pulley or electromagnetic clutch suitably used for driving a compressor for an air conditioning system, and more particularly to its torque limiting mechanism that interrupts torque transmission when the torque exceeds a predetermined torque level.
2. Description of the Related Art
Torque limiting mechanisms provide overload protection to drive source components such as belts by interrupting torque transmission when the driven side (i.e. driving) equipment is inhibited from rotating by a failure such as seizure of the sliding and rotating parts of a compressor.
One prior art example of a power transmission device having a torque limiting function is shown in Japanese Patent Laid-Open Publication No. Hei 8-319945. The device takes the form of a disk-like pulley having an annular groove formed with evenly spaced through holes so as to reduce the strength of the pulley. The groove breaks when the applied torque exceeds a predetermined torque level, thereby interrupting power transmission.
The method of interrupting power transmission by using a breakable part in the transmission path as with the above prior art example can be realized with a simple construction and offers the advantage of a low production cost. On the other hand, it involves difficulties in designing the breakable part for the following reasons.
The breakable part is designed to break when applied with a predetermined torque (referred to as “breakaway torque”) T1, while it is known that fatigue fracture occurs at a lower torque than the breakaway torque. Thus, the strength of the breakable part is set in consideration of its fatigue torque limit so that the peak torque during normal use should be set lower than the fatigue torque limit of the breakable part, as shown in FIG. 11.
In actuality, the torque limiting mechanism does not interrupt power transmission instantly when the preset torque limit is reached but sometime after the torque exceeds the limit. The range within which the torque limiting mechanism may be actuated is termed the “operation range” in this specification. The upper limit of this operation range is normally set to be a torque at which a malfunction occurs in the drive source components, an example of which is belt slippage.
However, in some cases depending on the material and shape of the breakable part, the average torque when the one-sided amplitude torque is zero, i.e., the average torque value at which the breakable part nearly always breaks may be higher than this upper limit of the operation range as can be seen from FIG. 11 (shown by a solid line).
This could be prevented by setting the torque limit line of the fatigue characteristics of the torque limiting mechanism as indicated by a dashed line in FIG. 11, so that the breakaway average torque does not exceed the upper limit of the operation range. However, this will greatly reduce the allowance between the peak torque during normal use and the fatigue limit of the breakable part, thus increasing the risk that the breakable part will break due to fatigue during normal use.